Infrared polarimetry of Mrk 231: scattering off hot dust grains in the central core
Siebenmorgen, R.; Telesco, C. M.; Packham, C.; Lopez-Rodriguez, E.; González-Martín, O.; Nikutta, R.; Levenson, N. A.; Alonso-Herrero, A.; Ramos Almeida, C.; Ichikawa, K.; Roche, P. F.; Perlman, E.; Jones, T. J.; Shenoy, D.; Martinez-Paredez, M.; Gordon, M. S.
United States, Japan, Germany, United Kingdom, Chile, Spain, Mexico
Abstract
We present high-angular (0.17-0.35 arcsec) resolution imaging polarimetric observations of Mrk 231 in the 3.1 μm filter using MMT-Pol on the 6.5-m MMT, and in the 8.7, 10.3, and 11.6 μm filters using CanariCam on the 10.4-m Gran Telescopio CANARIAS. In combination with already published observations, we compile the 1-12 μm total and polarized nuclear spectral energy distribution (SED). The total flux SED in the central 400 pc is explained as the combination of (1) a hot (731 ± 4 K) dusty structure, directly irradiated by the central engine, which is at 1.6 ± 0.1 pc away and attributed to be in the pc-scale polar region, (2) an optically-thick, smooth and disc-like dusty structure (`torus') with an inclination of 48° ± 23° surrounding the central engine, and (3) an extinguished (AV = 36 ± 5 mag) starburst component. The polarized SED decreases from 0.77 ± 0.14 per cent at 1.2 μm to 0.31 ± 0.15 per cent at 11.6 μm and follows a power-law function, λ∼0.57. The polarization angle remains constant (∼108°) in the 1-12 μm wavelength range. The dominant polarization mechanism is explained as scattering-off hot dust grains in the pc-scale polar regions.